1. Field of the Invention
The present invention generally relates to one-cylinder stack (OCS) capacitors and to methods of fabricating the same, and more particularly, the present invention relates to OCS capacitors which have been formed from a double-mold, and to methods of fabricating OCS capacitors using a double-mold.
2. Description of the Related Art
As semiconductor devices increase in memory cell density, the area occupied by capacitors of the memory cells has decreased. Capacitance is proportional to the dielectric constant of the capacitor dielectric and to the surface area of the capacitor electrodes. Thus, to increase capacitance, it is necessary to either select a dielectric of increased dielectric constant and/or to increase the surface area of the capacitor electrodes. However, adopting new dielectric materials is generally expensive and time-consuming in that it is often necessary to supply new manufacturing equipment, to verify the reliability of the dielectric materials, and to ensure the ability to mass produce devices having the dielectric materials. Therefore, increasing the surface area of electrodes is usually the most cost-effective means of satisfying a requirement for increased capacitance of a device having conventional dielectrics, such as dielectric (NO) layers composed of a stack of silicon nitride and silicon oxide layers.
Hemispherical grain (HSG) electrodes are used in conventional 128-Mbit or less DRAMs to increase the effective surface area of the electrodes. However, these electrodes cannot be applied in highly integrated devices of 256-Mbit or greater because any reduction in spacing between electrodes is limited due to the presence of HSGs on the surface of the electrodes. As such, in the case of a one-cylinder stack (OCS) storage electrode, increasing the height of the electrode from 1.4 μm to 1.6 μm, for example, has generally been considered the most efficient means of increasing the capacitance thereof.
The fabrication of a conventional OCS storage electrode will now be described with reference to FIGS. 1-6. In particular, an OCS capacitor storage electrode having a design rule of less than 1.2 μm is typically formed using a single mold. Referring to FIG. 1, an etch stop layer (not shown) is formed on an interlayer insulating layer 10 having a contact plug 15 in a surface thereof, and an oxide layer (not shown) is formed to a thickness h1, which substantially corresponds to the desired height of an electrode. Next, the oxide layer and the etch stop layer are sequentially etched into an oxide mold 30 having an underlying etch stop 20. In this manner, an opening 40 is defined over the contact plug 15, thereby exposing the contact plug 15. Then, as shown in FIG. 2, polysilicon is deposited on the sidewalls of the opening 40 and on the contact plug 15 so that a cylindrical electrode 50 is formed.
Reference is made to the scanning electron microscopic (SEM) photograph of FIG. 3. As mentioned previously, to increase the capacitance in the case of highly integrated devices, it is desired to increase the height of the OCS capacitor storage electrode to 1.6 μm or greater. However, noting that etching limitations set the slope of the sidewalls of the opening 40, if the height of the oxide mold layer is increased from h1 to h2 (about 1.8 μm) and a lower critical dimension (d) of the opening 40 is maintained, a bridge B (FIGS. 2 and 3) may be formed between neighboring OCS capacitor storage electrodes, thereby causing a twin-bit failure. In an effort to avoid twin-bit failures, the upper critical dimension (a) of the opening 40 can be reduced by reducing the lower critical dimension (d) of the opening 40. However, this approach is disadvantageous in that the surface area at the bottom of the opening 40 is reduced, thereby reducing capacitance.
It is also noted that twin-bit failures can occur as a result of storage electrodes falling down and contacting adjacent electrodes. As such, consideration must be given to the structural integrity of the electrodes.
That is, referring now to FIG. 4, after separation of the storage electrodes for cell isolation has been ensured, the oxide mold 30 is removed by wet etching. During this wet etching process, an etchant can permeate into the interface between each storage electrode 50 and an etch stop 20, thus etching the interlayer insulating layer 10. This can weaken the foundation of the storage electrode, causing the storage electrode to tip over and contact adjacent electrodes. FIG. 5A is a scanning electron microscopic (SEM) photograph showing an example in which falling electrodes have resulted in a depression having an @-shaped pattern, and FIG. 5B is a scanning electron microscopic (SEM) photograph showing a cross-sectional side view of electrodes falling into one another.
Therefore, there is an increasing need for a new OCS capacitor structure and a method for fabricating the same in which depressions and bridging between adjacent electrodes can be effectively avoided even in the case where the capacitor height is on the order of 1.6 μm or more.